Methods for non-destructive testing of ferro-magnetic material (i.e. a "work piece") for flaws, defects and other discontinuities through the use of fluorescent magnetic particles have been described in a number of patents, for instance, U.S. Pat. Nos. 3,404,093 and 3,485,758. Conventional methods involve applying the fluorescent magnetic particles to the surface of the work piece, magnetizing the work piece and then inspecting the work piece under filtered ultraviolet light (also known as "black light") for characteristic patterns formed by particles which have migrated to the flaws. One of the advantages of using fluorescent magnetic particles for inspection purposes, as compared with magnetic particles having only visible coloration, is that there is a greatly increased contrast between the fluorescent magnetic particles and the background of the work piece, thereby easing the detection of flaws and the like.
Certain techniques for combining magnetic particles and fluorescent pigments are known in the art. See U.S. Pat. No. 2,365,253. One such technique involves incapsulation of pigment and magnetic particles to form a fluorescent pigment/magnetic particle composite. But conventional technology has certain drawbacks which significantly detract from its usefulness, especially in automatic testing applications.
Illustratively, consider the subject matter of U.S. Pat. No. 3,485,758, directed to a method of making fluorescent magnetic particles which involves encapulating a core including ferro-magnetic material and fluorescent pigment particles with a film-forming resin. Encapsulation is achieved by adding the resin to a mixture of pigment and ferro-magnetic material powders. Unfortunately, the patented method simply does not provide sufficient encapsulation, and thus a significant amount of pigment powder particles remain loose and unencapsulated. The presence of these loose pigment particles results in "background" fluorescence which interferes with the testing procedure ultimately performed on the work piece. The background fluorescence detracts from the contrast which is sought between fluorescent magnetic particles and the work piece background, thereby impeding the inspection of the surface of the work piece for flaws discontinuities, and other defects. Removal of the background fluorescence by screening of the material through a sieve has proven to be unsuccessful; due to the small size of the particles involved, they pass through the screen. Another method of removing background fluorescence by utilizing a magnetic field to separate out the magnetic particles has been found to be extremely inefficient and expensive.
A further disadvantage associated with conventional embodiments is that a lesser amount of pigment is encapsulated in each particle, the level of brightness of the desired fluorescence is lower and therefore difficulties from the lack of contrast are aggravated.